Novel strategies are needed when standard approaches fail to delineate the mechanism by which a mutation causes disease. This is the case for the severe, degenerative joint disease that occurs in patients with Progressive Pseudorheumatoid Arthropathy of Childhood (PPAC) caused by mutations in WISP3. Many WISP3 loss-of-function alleles have been identified in large numbers of PPAC patients. Teenage PPAC patients require total hip and knee replacement surgery for end-stage articular cartilage failure. However, at the time joints are replaced, PPAC cartilage is indistinguishable from common end-stage osteoarthritic cartilage. Because PPAC becomes symptomatic during mid-childhood and then rapidly progresses to end- stage joint failure, it is ethically difficult to ask parents for permission to collect their child's cartilage before symptoms appear or early during the degenerative process. Thus, patient samples have only been collected at the time of arthroplasty and have failed to elucidate why WISP3 deficient cartilage fails precociously. Mice lacking Wisp3 have also failed to identify the gene's biologic function. No skeletal phenotypes have been detected in 2 different knockout mice and 2 different Wisp3 overexpressing mice. Additionally, in vitro studies in a variety of cultured cells using a variety of assays have suggested multiple biologic activities for WISP3; yet the in vivo or cartilage-specific relevance of these findings is uncertain. Therefore, to better understand PPAC, with support from the Charles H. Hood Foundation, we generated induced pluripotent stem cells (iPSCs) from 5 patients with PPAC (WISP3 deficient), and with CRISPR/Cas9 gene editing we are making them WISP3 sufficient. With R21-funding, we intend to determine how WISP3 deficiency causes disease by differentiating iPSCs into articular and growth plate chondrocytes. We will compare cartilages produced from isogenic mutant and WISP3-corrected iPSCs histologically, mechanically, biochemically, and transcriptomically to search for consistent differences that will inform us about WISP3 function. Knowledge we gain will benefit patients with PPAC and could point to new approaches for protecting cartilage from common degenerative joint disorders such as age-related and post-traumatic osteoarthritis. Other laboratories will be interested in our WISP3 deficient and corrected cell lines, and in the methods and analytic approaches we develop that use iPSCs to study cartilage growth and homeostasis.